This invention relates to a method and apparatus for copy protection of a video signal. More particularly, the present invention relates to modifying a video signal so it can be satisfactorily displayed on a receiver, while recorded copies of the modified video signal cannot be satisfactorily displayed.
A video signal may be in a format according to one of several standards. The standard conventionally used in the United States is the NTSC standard, which specifies that a field of information is transmitted every 1/60 of a second and each field is organized into 262.5 lines of information. A frame consists of two fields. The first 20 lines of each field are referred to as the vertical blanking interval, and are devoid of video information. The first three line intervals of the vertical blanking interval contain a standard sequence of pulses used for vertical synchronization, namely, a sequence of equalizing pulses in lines 1-3, followed by a sequence of vertical sync pulses in lines 4-6, followed by another sequence of equalizing pulses in lines 7-9, thereby indicating the start of a field. Lines 10-20 each contain a horizontal blanking interval and are otherwise devoid of information. The remaining so-called active lines in each field contain useful video information, that is, information to be displayed on a television receiver. As is conventional, the active line intervals begin with a horizontal blanking interval, and this blanking interval is followed by a waveform to be displayed which represents a line of an image. The horizontal blanking interval is also referred to herein as the horizontal synchronizing interval.
As can be seen from FIG. 1, the horizontal blanking interval includes a waveform portion having an amplitude of 0 IRE units, referred to as a "front porch", followed by a waveform portion having an amplitude of -40 IRE units, referred to as a horizontal sync pulse. The horizontal sync pulse is followed by another waveform portion having an amplitude of 0 IRE units, referred to as a "breezeway", and the last portions of the horizontal blanking interval are the color burst, and the color back porch.
The prior art includes techniques for modifying a video signal so it can be satisfactorily displayed on a receiver, while recorded copies of the modified video signal cannot be satisfactorily displayed.
One such technique adds a large amplitude pulse to the back porch of the horizontal blanking interval. A variation of this technique alternately adds one of two pulses to the back porch. These pulses are eliminated by a chroma signal filter in a television, but impair the operation of the automatic gain control (AGC) circuit in a videocassette recorder (VCR) so that the signal recorded by the VCR cannot be satisfactorily displayed.
Another such technique adds an interference signal including a narrow positive pulse near the leading edge of the horizontal sync pulse, and a negative pulse of approximately equal area to the front porch. The interference signal interferes with successful color phase restoration on playback of a copy of a video signal which includes the interference signal.
However, none of these techniques has been found to result in a displayed copy of the video signal which is sufficiently unsatisfactory. Further, none of these techniques modifies a video signal so as to prevent detection of its horizontal sync pulses during display of a copy of the modified video signal.